The present invention is directed to a method for measuring the optical attenuation of an optical medium upon employment of two measurement transmitters and two receivers that are arranged at both sides of the optical medium, wherein the transmitters and receivers are each, respectively, coupled to the light waveguide and measurements are implemented proceeding from both sides of the optical medium.
Measuring methods, wherein a flex coupler is employed, are disclosed, for example, in U.S. Pat. No. 4,652,123, whose disclosure is incorporated herein by reference thereto and in allowed U.S. patent application Ser. No. 07/394,114, filed Aug. 15, 1989; which issued as U.S. Pat. No. 5,078,489 on Jan. 7, 1992, whose disclosure is incorporated herein by reference thereto and which claims priority from German Application 38 28 604. As presented in detail in these two documents, particularly exact results can be achieved when work is carried out with two transmitters and two receivers and a plurality of measurements are successively implemented proceeding from both sides of the test subject.
These and all similar measuring methods are based on the measuring principle that the coupling of the receiver or receivers and of the transmitter or transmitters occurs on the basis of at least one so-called flex coupler, wherein the light waveguide experiences a deformation in the measurement region. The light emerging in the deformation region is acquired at the receiver and is offered for evaluation in the measuring equipment. The light is picked up by a rigidly arranged light receiver, for example a photodetector, and is offered for evaluation. The deformed region of the light waveguide at the transmitter is located in the radiation field of a light source, for example a light emitting diode. The quantity of the incoupled or infed light and the outcoupled or outfed light, respectively, is dependent, first, on the spatial allocation of the light waveguide proceedingly bent in the coupling region vis-a-vis the light receiver and also on the refractive index ratio and on the geometry of the optical boundary surfaces in the flex coupler insofar as no matching of refractive index occurs, either by immersion or elastic material. In and of itself, the measuring instrument is already operational immediately after the mechanical actuation of the flex coupler, for example after the closing of a cover, and the measurements could be immediately begun.
It has been shown, however, that more and more inprecise values are obtained the earlier the measurement is carried out after bending of the light waveguide into the coupling position. The investigations have led to the conclusion that a chronological variation of the infeed or, respectively, outfeed efficiency apparently occurs, namely in that a deformation or, respectively, a flowing of the coating of the light waveguide occurs in the coating of the light waveguide when subjected to the pressure element that effects the mechanical bending of the light waveguide, for example when subjected to the arbor, bending beam or the like. This flowing, however slight the modification, will change the position of the deformed region with respect to the light source or with respect to the light receiver and also influences the geometry of the optical boundary surface. Such error influences could, in fact, be eliminated in that one waits until the deformation of the coating has ended because the final condition of the radiation field in the direction onto the photodetector has been reached and a chronological change of the measured signals can no longer be identified, given a constant transmission signal.